Apparatus for purification of exhaust gas and method for purification of exhaust gas using the same

Abstract

An apparatus for purification of exhaust gas comprises: an exhaust gas pipe (2) connected to an internal combustion engine (1); an upstream catalyst (3) arranged at the upstream side of an exhaust gas passage inside the exhaust gas pipe (2); a downstream catalyst (4) which is arranged at the downstream side of the exhaust gas passage inside the exhaust gas pipe (2), and on which nickel and/or iron is supported; and oxidizing gas supply means (5) which is connected to the exhaust gas pipe (2) between the upstream catalyst (3) and the downstream catalyst (4), and which supplies oxidizing gas into the exhaust gas pipe (2).

Description

TECHNICAL FIELD[0001]The present invention relates to an apparatus for purification of exhaust gas and a method for purification of exhaust gas using the apparatus.BACKGROUND OF THE INVENTION[0002]Catalysts for purifying exhaust gas from a gasoline engine, such as so-called NOx storage catalysts and three-way catalysts, have been put into practical use.[0003]Among these catalysts, NOx storage catalysts generally have a function to store NOx included in exhaust gas from an internal combustion engine when the air-fuel ratio of the exhaust gas is a lean state, and to release the stored NOx for reduction when the air-fuel ratio of the exhaust gas from the internal combustion engine is a theoretical (stoichiometric) or rich state. Such catalysts also have the nature of storing sulfur (S) components, in addition to NOx, included in the exhaust gas in a form of sulfate. Being highly stable compared to NOx, and impossible to be released from a catalyst simply by shifting an air-fuel ratio o...

AI Technical Summary

Benefits of technology

[0029]Although it is not clearly understood why the above-described objects are achieved by using the apparatus for purification of exhaust gas and the method for purification of exhaust gas of the present invention, the inventors estimate the reasons as follows. Firstly, in the present invention, the oxidizing gas supply means is connected between the upstream catalyst and the downstream catalyst, and thereby the oxidizing gas is allowed to be added between the upstream catalyst and the downstream catalyst. Thus, it is possible to rapidly shift the air-fuel ratio at the downstream side to a stoichiometric or lean state while the air-fuel ratio at the upstream side is maintained to be in a rich state. Since the air-fuel ratio of the downstream side can be rapidly shifted to a stoichiometric or lean state, when it becomes necessary to restrain the generation of H2S due to, for example, a rapid change in the operation condition of the internal combustion engine, it is possible to reliably restrain the generation of H2S by temporarily supplying oxidizing gas. Therefore, in the present invention, by preventing thermal deterioration of upstream catalyst in a high-temperature lean atmosphere while sufficiently avoiding sulfur poisoning of the upstream catalyst, it is possible to reliably restrain the generation of H2S. Furthermore, in the present invention, adopting the oxidizing gas supply means, it is capable of sufficiently restraining the generation of H2S without being affected by the oxygen storage capacity of the upstream catalyst.

[0030]The downstream catalyst used in the present invention which is a catalyst on which nickel (Ni) and/or iron (Fe) is supported. Thus, it is possible to sufficiently desorb sulfur components in a form of SO2 even when the air-fuel ratio is in a stoichiometric state (refer to FIG. 2). When the air-fuel ratio is in a stoichiometric or lean state, the reaction between the desorbed SO2 and hydrogen in exhaust gas is inhibited, and thereby the generation of H2S originating from the desorbed SO2 is also restrained. For this reason, when the air-fuel ratio of the exhaust gas flowing into the downstream catalyst is in a stoichiometric or lean state, it is possible to sufficiently restrain the generation of H2S while the sulfur components are sufficiently desorbed in a form of SO2. In the present invention, utilizing a combination of the oxidizing gas supply means and the downstream catalyst, it is possible to more reliably restrain the generation of H2S in accordance with a rapid change in the operation condition.

[0031]In this section, discussed will be the case where a conventional apparatus for purification of exhaust gas adopts an automobile engine as an internal combustion engine. Accumulating, within a catalyst, sulfur components in exhaust gas at a low temperature, and desorbing the accumulated sulfur components under a high-temperature rich state in general, the downstream catalyst, which is often exposed to a low temperature of 500° C. or below, tends to generate H2S (exhaust odor). Especially, in the case where the operation condition rapidly changes, such as hill-climbing or high-speed driving following low-speed driving on city streets, the air-fuel ratio of the exhaust gas flowing into the downstream catalyst is rapidly shifted to a high-temperature rich state. As a result, exhaust odor is likely to be generated. Similarly, in the case where an automobile is brought to a stop after a short-term hill-climbing or high-speed driving, the air-fuel ratio of the exhaust gas flowing into the downstream catalyst is also rapidly shifted to a high-te...

Problems solved by technology

Therefore, such a catalyst may have caused a problem that NOx storage capacity is decreased due to the accumulation of sulfur components (such a phenomenon has been referred to as “sulfur poisoning (S poisoning)”).

Therefore, a large amount of SOx is emitted in a regeneration process of the catalyst.

However, the apparatuses for purification of exhaust gas described in Documents 1 to 5 are not capable of rapidly shifting the air-fuel ratio of the exhaust gas flowing into the catalyst at the downstream side to a lean state.

Accordingly, such apparatuses are unable to respond to a rapid change in the operation condition of an internal combustion engine, causing highly-concentrated H2S to be temporarily emitted in some cases.

Thus, when the atmosphere of the exhau...

Images